Fiber Bragg Gratings (FBG) are widely used in various optical sensing and communications applications. FBG, prepared by well-known methods, reflect a selected design wavelength transmitting input light with a notch at the design wavelength. There is significant interest in devices that will reliably allow the reflection wavelength of an FBG to be tuned. While it has been recognized that the reflection wavelength of an FBG can be tuned by stretching or compressing the waveguide (typically an optical fiber) in which the FBG is written, mechanical methods that have been applied to this problem most often result in devices with relatively narrow tuning ranges or when wider tuning is attempted in frequent fiber breakage. These methods have generally not been practical in application.
U.S. Pat. No. 5,007,705 relates generally to methods for tuning fiber Bragg gratings by temperature change, electromechanical stretching or by application of fiber coatings that are sensitive to modulating electrical, magnetic or acoustic fields. In a specific example, tuning is reported to be achieved by winding a fiber containing a Bragg grating around the circumference of a piezoelectric cylinder. The circumference of the cylinder expands on application of a voltage, the fiber is stretched and the wavelength of the Bragg grating is changed.
U.S. Pat. No. 5,469,520 reports tuning of a Bragg grating up to 45 nm by compression of the fiber carrying the grating using a moving piston actuated by a stepping motor. The fiber is retained in sliding ferrules to minimize fiber buckling. U.S. Pat. No. 6,229,827 also reports tuning of a fiber grating by compression of the fiber. The fiber containing the grating is encased in glass to form a tunable element which is axially (along the fiber axis) compressed. Encasing the fiber in glass is described as preventing buckling on compression. WO 00/39617 relates to a tube-encased fiber grating.
U.S. Pat. No. 6,240,220 and International Application WO/00/07047 report tuning of fiber gratings by varying longitudinal displacement of a support member supporting a fiber carrying the grating to uniformly stretch the fiber. In a specific example, a convex fiber support member having a channel on that surface for receiving the fiber is illustrated. The fiber support member in this example is displaced normal to a portion of the fiber in the channel. U.S. Pat. Nos. 5,999,671; 5,991,483; 5,841,920; 5,602,949 and 5,367,589 also relate to tuning of fiber gratings.
Various methods have been employed to temperature-compensate fiber gratings. U.S. Pat. No. 6,044,189, for example, describes a temperature compensation structure to which the fiber carrying the grating is bonded. The structure has two plates made of materials having temperature coefficients of expansion and the fiber is bonded to the external surface of the plate having the lower temperature coefficient. The structure bends with temperature change and elongates the fiber as temperature decreases. U.S. Pat. Nos. 6,181,851; 5,042,898; 5,978,539; and 5,694,503 also relate to temperature compensation of fiber gratings.
The present invention relates to devices and methods for tuning fiber gratings by bending the fiber containing a fiber grating. A desirable wavelength tuning range of about 50 nm can be achieved, for example, by compressing a fiber by about 5%. In principle, this amount of compression can be achieved by bending a fiber along its axis, however, transverse forces that would accompany mechanical bending must be minimized or the fiber will break. This invention provides devices and methods for tuning the reflection wavelength of fiber gratings to achieve a desirable tuning range while minimizing fiber breakage.
A fiber containing a fiber grating is uniformly, precisely and reproducibly bent by embedding the fiber in a substrate or mounting the fiber on a substrate parallel to the longitudinal (long) axis of the substrate and applying appropriate force to bend the substrate perpendicular to the length of the fiber, i.e., perpendicular to the longitudinal axis of the substrate. Further, embedding of the fiber carrying the fiber grating parallel to, but offset from the plane containing the central axis which is normal to the bend of the substrate allows bending of the substrate along that central axis of the substrate to achieve enhanced fiber compression or stretching dependent upon the direction of bending of the substrate relative to the direction the fiber is offset from the central axis.
The substrate in which the fiber is encased or embedded must be sufficiently wider than the fiber (the substrate width is much greater than the fiber diameter) so that bending of the substrate uniformly compresses or stretches the fiber. The substrate must also be sufficiently elastic and resilient that it can be repeatedly bent without being deformed, returning to its original unbent shape when bending forces are removed. Sufficient elasticity can be achieved by choice of material and material thickness.
In specific embodiments, the fiber is embedded in a substrate offset in thickness (the smallest dimension of a rectangular substrate) from the central longitudinal axis of the substrate. The substrate can be formed of a single component or uniform mixture of components or it can be formed from two or more layers or a laminate of the same or different materials.
In other specific embodiments, the fiber is mounted on a substrate surface by application of a layer of adhesive which bonds the fiber at least along the length of the fiber grating to the substrate surface. The substrate itself in this embodiment may be composed of a single layer or multiple layers of different materials.
The present invention is directed to substrates containing one or more fibers carrying fiber gratings, particularly to fibers carrying FBGs, wherein the fiber is offset from the plane of containing the central axis of the substrate that is normal to the bend. The invention is also directed to fiber grating tuning devices in which the substrate carrying the offset fiber is bent with a substantially uniform radius over substantially the entire length of the fiber grating. The invention is further directed to optical devices which employ the tuning devices of this invention and to methods of tuning the wavelength of an fiber grating (particularly a fiber Bragg grating) using the bending procedure, the substrate or the tuning devices of this invention.
More specifically, the invention relates to a fiber grating tuning device in which an optical fiber carrying a fiber grating is longitudinally mounted upon or longitudinally embedded within the substrate such that the fiber axis is offset from the plane of the substrate that contains the central longitudinal axis of the substrate and which is normal to bend that is to be applied. The substrate will normally be bent normal to its length and parallel to its smallest dimension (its thickness). The geometry of the bend and its relationship to the offset is illustrated in FIGS. 1A-1K herein. The tuning device also contains an actuator for application of force to the substrate to bend it perpendicular (or normal) to the longitudinal axis of the substrate.
The substrate of the tuning device can comprises two or more layers of different materials wherein the fiber carrying the fiber grating is embedded between any two layers of the substrate. In this configuration the fiber may be embedded within the substrate within a layer of adhesive. The tuning device can comprise a fiber mounted lengthwise (longitudinally) upon a surface of the substrate in an adhesive layer.
In specific embodiments of this invention the substrate is bent by mounting the substrate directly or indirectly on the surface of a cylinder, loop or ring and changing the diameter of the loop or cylinder to bend the substrate. The cylinder, loop or ring functions to convert force applied to it to change its diameter to bend the substrate. A specific actuator comprises a loop made of resilient material having two ends wherein the diameter of the loop is changed by application of force (pulling or pushing) to one or both ends of the loop. In this specific embodiment the substrate can be mounted on an outside surface or an inside surface of the loop. In another specific embodiment, the actuator is a cylindrical piezoelectric transducer upon which the substrate is mounted and which changes its diameter on application of a voltage to the piezoelectric transducer. In yet another specific embodiment, the actuator comprises a cylinder upon which the substrate is mounted in a relaxed or unbent state such that the substrate longitudinal axis is parallel to the longitudinal axis of the cylinder and wherein a uniform bend is applied to the substrate by winding the substrate around the cylinder. Winding can be achieved in this embodiment, for example, by bonding the substrate to a winding strip attached at either longitudinal end to moveable elements which can be rotated around the cylinder. The winding strip and the substrate mounted upon it is wound around the cylinder by rotating the moveable elements in opposite directions relative to each other around the cylinder.
The invention also relates to a method for tuning a fiber grating employing the tuning devices of this invention. More generally the method involves applying force to a substrate of this invention which carries a fiber and a fiber grating to bend the substrate perpendicular to longitudinal axis of the substrate and parallel to the direction the fiber axis is offset from the central longitudinal axis of the substrate. The bend is preferably applied to the substrate such that the radius of the bend uniformly increases or decreases at least along the length of the fiber grating in the substrate. In specific embodiments the substrate of this invention is bent by changing the diameter of a cylinder, loop or ring to which the substrate is directly or indirectly attached.
The invention is further illustrated and exemplified in the following description and drawings in which similar features are identified by like numbers.